1. Field of the Invention
The present invention relates to an active matrix substrate used in a liquid crystal display device or the like and a method for driving the same.
2. Description of the Related Art
Hereinafter, a conventional active matrix substrate will be described referring to FIGS. 3 and 4. FIG. 3 shows a plan view of a conventional active matrix substrate, end FIG. 4 shows an equivalent circuit thereof. Two thin film transistors (TFTs) 303a and 303b are used for a switching function, and are controlled so as to be turned to an ON-state or OFF-state by a gate signal supplied from a gate driving circuit 309 via a gate bus line 301. A source driving circuit 308 supplies a video signal to a source bus line 302. The two TFTs 303a and 303b are connected in series. Such a configuration has an advantage of reducing the OFF-current in the TFTs over a single TFT. The OFF-current means a leakage current occurring between the source electrode and the drain electrode of the TFT when the TFT is in the OFF-state. The source bus line 302 is connected To the source electrode of the TFT 303a through a contact hole 305. The drain electrode of the TFT 303b is connected to a pixel electrode 304 through a contact hole 306. The pixel electrode 304 is formed of a transparent conductive film such as an Indium Tin Oxide (ITO) film. When the electric potential of the gate bus line 301 becomes high so as to turn ON TFTs 303a and 303b, the video signal is written into a pixel 307. The capacitance of the pixel 307 is equivalent to a capacitance obtained by sandwiching a liquid crystal between the pixel electrode 304 and a counter electrode on a counter substrate. The written signal Is held when the TFTs 303a and 303b are in the OFF-state. The characteristics of the pixel 307 for holding the video signal can be improved by providing an additional capacitance in parallel with the pixel 307 on the active matrix substrata.
Thus, in the conventional active matrix substrate, a plurality of TFTs arranged in series are used for a switching function in order to reduce leakage current in the TFTs. However, according to such a configuration, gate electrodes of the TFTs are electrically connected to one another. As a result, the same electric potential is applied to the respective gate electrodes of the TFTs.
In an active matrix type liquid crystal display device, a video signal written into a pixel is held by keeping the OFF-state of TFTs while the other video signals are written into the other pixels, thereby realizing a good display quality. When the OFF-characteristics of the TFT are insufficient, the video signal may be attenuated, and thus good display quality cannot be obtained. Herein, the OFF-characteristics mean characteristics of the TFT when the TFT is in the OFF-state. One method for improving the characteristics of the pixel for holding the video signal includes the step of providing an additional capacitance in parallel with the pixel on the active matrix substrate so as to increase the capacitance for holding the video signal. However, this method has a disadvantage of reducing the numerical aperture of the liquid crystal display device when too large additional capacitance is provided, resulting in darkening the display. Conventionally, even when a plurality of TFTs arranged in series are used as a switching function in order to reduce leakage current in the TFTs, the OFF-characteristics of the TFTs cannot be improved sufficiently.